The present invention relates to variable speed AC motor drives for glassware forming machines, and more particularly, to an improved digital regulator for inverter drives.
It is accepted practice to use AC inverter drives as power sources for motors for various elements of a glassware manufacturing line including glass gob feeders, gob distributors, conveyors, etc. It is typical in such apparatus to drive these elements in synchronism, and for this purpose it is common practice to employ AC synchronous motors. These motors have well-defined speed-to-frequency characteristics, and such systems employ AC inverters having variable frequency outputs. The usual prior art controller for said A.C. inverter drives relies upon an analog output from a computer to regulate the frequency of the inverter signal, or allows manual control using one or more thumbwheels to vary the frequency. Analog systems suffer frequency drift due to temperature and voltage changes, and will lose the drive command in the event of a controller failure.
Another such system is shown in prior art U S. Pat. No. 4,007,028 to Bublitz et al. This system includes a common frequency control which is set up manually using a plurality of decade switches, such a thumbwheel switches, to vary the output frequency of a reference source. With reference to FIG. 1 of this reference, adjusting the decade switches 28 changes the output frequency of inverters 21-23, and hence simultaneously changes the speed of various drive motors. Such system further includes individual frequency control circuits each comprising a digital regulator and multiplier to vary a multiplication factor of individual multiplier circuits. The manual adjusting means for such multipliers again takes the form of decade switches, such as thumbwheel switches. These multiplier circuits, in effect, serve as "electronic gear boxes". This digital control apparatus risks the loss of the control inputs from panel 28 causing a loss of the reference frequency signal from source 27.
Accordingly, it is a primary object of the invention to provide an improved regulator unit for an AC inverter drive of a glassware forming machine. Subsidiary objects include achieving a low-cost, easy to install system.
Another object of the invention is to provide a more dependable control system than one which depends upon digital thumbwheel controls.